The present invention relates to certain poly(alkyl (meth)acrylates) useful as viscosity index improving additives for hydraulic fluids.
Hydraulic systems, e.g., systems wherein the operation of high speed, high pressure hydraulic pumps is subject to wide temperature variations, can impose severe demands on hydraulic fluids.
Additives for improving the properties, e.g., the viscosity index, of hydrocarbon base oils used in hydraulic fluids are known. Viscosity index improving additives reduce the influence of temperature changes on fluid viscosity. British Patent GB 1,172,697 discloses viscosity index improving copolymers of up to 50 weight % xe2x80x9creadily polymerizablexe2x80x9d monoethylenically unsaturated monomers, e.g., styrene, t-butyl methacrylate, methyl methacrylate and mixtures thereof, with at least 50 weight % xe2x80x9cdifficulty polymerizablexe2x80x9d monoethylenically unsaturated monomers, e.g., lauryl methacrylate. U.S. Pat. No. 5,112,509 discloses a method for making a poly(methyl methacrylate-co-lauryl methacrylate) copolymer for use in hydraulic fluids and lubricating oil compositions as a viscosity index improver. The process includes heating a reaction mixture of the monomers and a polymerization initiator to a temperature from 200xc2x0 F. to 300xc2x0 F.
Paraffinic oils have a tendency to gel at low temperatures due to ordering of wax molecules in the oil. In some hydraulic systems, e.g., mobile equipment, startup temperatures may be well below 0xc2x0 F. and it is critically important that the hydraulic fluid in the system remains fluid at the low temperatures encountered. High performance hydraulic fluid compositions for applications involving low startup temperatures typically include a pour point depressing additive, in addition to a viscosity index improving additive, to improve the low temperature fluidity of the hydraulic fluid.
Along with their several advantageous effects, known poly(alkyl (meth)acrylate) viscosity index improvers can impart at least one undesirable property to the hydraulic fluids in which they are used. Hydraulic fluids formulated with such additives have shown a tendency to form emulsions with ambient moisture during use. The performance of the emulsified fluids is compromised with respect to, e.g., lubricity, corrosion resistance, low temperature performance and compressibility. In addition, the presence of small amounts water in hydraulic fluids has been found to detrimentally effect the filterability of such fluids. Reduced filterability may result in plugging of hydraulic system filters.
A method for making a copolymer for improving the viscosity index of a hydraulic fluid is disclosed. The method includes the steps of:
heating a reaction mixture to a reaction temperature from about 75xc2x0 C. to about 100xc2x0 C., said reaction mixture comprising:
from about 55 weight percent to about 99.5 weight percent of a first monomer selected from the group consisting of (C8-C5)alkyl (meth)acrylates and mixtures thereof;
from about 0.5 weight percent to about 45 weight percent of a second monomer selected from the group consisting of (C1-C7)alkyl (meth)acrylates, (C16-C24)alkyl (meth)acrylates and mixtures thereof;
an effective amount of a polymerization initiator; and
a hydrocarbon diluent; and
maintaining the reaction mixture at the reaction temperature for a period of time effective to allow copolymerization the monomers. Copolymers made by the process of the present invention provide viscosity index improvement to the hydraulic fluids in which they are used while providing improved demulsibility and filterability relative to viscosity index improving additives made by known processes.
The copolymer of the present invention includes from about 55 weight percent (wt %) to about 99.5 wt % repeating units, each having the structural formula (1): 
wherein each occurrence of R1 is H or methyl and each occurrence of R2 is independently selected from the group consisting of (C8-C15)alkyl and from 0.5 wt % to about 45 wt % repeating units selected from the group consisting of repeating units of the structural formulae (2), (3) or (4) disclosed below, and mixtures thereof. As used herein, the term xe2x80x9ccopolymerxe2x80x9d means a polymer having more than one type of repeating unit and includes, e.g., copolymers, terpolymers and tetrapolymers.
Preferably, R1 is methyl.
As used herein, (C8-C15) alkyl means any straight or branched alkyl group having 8 to 15 carbon atoms per group, e.g., octyl, nonyl, decyl, isodecyl, undecyl, lauryl, tridecyl, myristyl, pentadecyl. Preferably, R2 is(C10-C15)alkyl. More preferably, R2 is selected from the group consisting of isodecyl, lauryl, tridecyl, myristyl, pentadecyl and mixtures thereof.
The copolymer of the present invention includes from about 0 wt % to about 45 wt % repeating units, each having the structural formula (2): 
wherein each occurrence of R3 is independently H or methyl and each occurrence of R4 is independently n-butyl, isobutyl or t-butyl.
The copolymer includes from about 0 wt % to about 20 wt % repeating units, each having the structural formula (3): 
wherein each occurrence of R5 is independently H or methyl. Preferably, R5 is methyl.
The copolymer includes from about 0 wt % to about 35 wt % repeating units, each having the structural formula (4): 
wherein each occurrence of R7 is independently H or methyl and each occurrence of R8 is independently selected from the group consisting of (C16-C24) alkyl.
Preferably, R7 is methyl.
As used herein, (C16-C24) alkyl means any straight or branched alkyl group having 16 to 24 carbon atoms per group, e.g., stearyl, heptadecyl, cetyl, nonadecyl, eicosyl. Preferably, and R8 is (C16-C20)alkyl. More preferably, R8 is selected from the group consisting of stearyl, cetyl, eicosyl and mixtures thereof.
The copolymer of the present invention has a number average molecular weight, determined, e.g., by gel permeation chromatography, between about 15,000 and about 120,000, preferably between about 20,000 and about 100,000, and most preferably between about 25,000 and about 75,000.
The copolymer of the present invention has a weight average molecular weight, determined, e.g., by gel permeation chromatography, between about 25,000 and about 225,000, preferably between about 37,500 and about 225,000, and most preferably between about 50,000 and about 200,000.
In a preferred embodiment, the copolymer includes from about 55 wt % to about 99.5 wt % repeating units of the structural formula (1) and from about 0.5 wt % to about 45 wt % repeating units of the structural formula (2). More preferably, the copolymer includes from about 60 wt % to about 90 wt % repeating units of the structural formula (1) and from about 10 wt % to about 40 wt % repeating units of the structural formula (2). Even more preferably, the copolymer includes from about 70 wt % to about 85 wt % repeating units of the structural formula (1) and from about 15 wt % to about 30 wt % repeating units of the structural formula (2).
In a second preferred embodiment, the copolymer includes from about 55 wt % to about 98.5 wt % repeating units of the structural formula (1) from about 0.5 wt % to about 44 wt % repeating units of the structural formula (2) and from about 1 wt % to about 20 wt % repeating units of the structural formula (3). More preferably, the copolymer includes from about 60 wt % to about 87.5 wt % repeating units of the structural formula (1) from about 10 wt % to about 37.5 wt % repeating units of the structural formula (2) and from about 2.5 wt % to about 17 wt % repeating units of the structural formula (3). Still more preferably, the copolymer includes from about 70 wt %, to about 80 wt % repeating units of the structural formula (1) from about 15 wt % to about 25 wt % repeating units of the structural formula (2) and from about 5 wt % to about 15 wt % repeating units of the structural formula (3)
In a third preferred embodiment, the copolymer includes from about 55 wt % to about 97 wt % repeating units of the structural formula (1) and from about 0.5 wt % to about 42.5 wt % repeating units of the structural formula (2) and from about 2.5 wt % to about 35 wt % repeating units of the structural formula (4). More preferably, the copolymer includes from about 60 wt % to about 85 wt % repeating units of the structural formula (1) and from about 10 wt % to about 35 wt % repeating units of the structural formula (2) and from about 5 wt % to about 25 wt % repeating units of the structural formula (4). Still more preferably, the copolymer includes from about 65 wt % to about 75 wt % repeating units of the structural formula (1) and from about 15 wt % to about 25 wt % repeating units of the structural formula (2) and from about 10 wt % to about 20 wt % repeating units of the structural formula (4).
In a fourth preferred embodiment, the copolymer includes from about 55 wt % to about 96 wt % repeating units of the structural formula (1), from about 0.5 wt % to about 41.5 wt % repeating units of the structural formula (2), from about 1 to about 20 wt % repeating units of the structural formula (3), and from about 2.5 wt % to about 35 wt % repeating units of the structural formula (4). More preferably, the copolymer includes from about 55 wt % to about 82.5 wt % repeating units of the structural formula (1), from about 10 wt % to about 37.5 wt % repeating units of the structural formula (2), from about 2.5 wt % to about 17 wt % repeating units of the structural formula (3), and from about 5 wt % to about 25 wt % repeating units of the structural formula (4). Still more preferably, the copolymer includes from about 55 wt % to about 70 wt % repeating units of the structural formula (1), from about 15 wt % to about 30 wt % repeating units of the structural formula (2), from about 5 wt % to about 15 wt % repeating units of the structural formula (3), and from about 10 wt % to about 20 wt % repeating units of the structural formula (4).
In a fifth preferred embodiment, the copolymer includes from greater than about 90 wt % to about 99.5 wt % repeating units of the structural formula (1) and from about 0.5 wt % to less than about 10 wt % repeating units of the structural formula (3). More preferably, the copolymer includes from about 92 wt % to about 97.5 wt % repeating units of the structural formula (1) and from about 2.5 wt % to about 8 wt % repeating units of the structural formula (3).
In a sixth preferred embodiment, the copolymer includes from about 55 wt % to about 96.5 wt % repeating units of the structural formula (1), from about 1 wt % to about 20 wt % repeating units of the structural formula (3) and from about 2.5 wt % to about 35 wt % repeating units of the structural formula (4). More preferably, the copolymer includes from about 60 wt % to about 92.5 wt % repeating units of the structural formula (1), from about 2.5 wt % to about 17 wt % repeating units of the structural formula (3) and from about 5 wt % to about 25 wt % repeating units of the structural formula (4). Still more preferably, the copolymer includes from about 70 wt % to about 85 wt % repeating units of the structural formula (1), from about 5 wt % to about 15 wt % repeating units of the structural formula (3) and from about 10 wt % to about 20 wt % repeating units of the structural formula (4).
The copolymer of the present invention can be made by free radical initiated polymerization of alkyl (meth)acrylate monomers, wherein the term xe2x80x9calkyl (meth)acrylatexe2x80x9d is used to refer to alkyl acrylate monomers, alkyl methacrylate monomers and mixtures thereof. Similarly, the terminology xe2x80x9c(meth)acrylic acid is used herein to refer to acrylic acid, methacrylic acid and mixtures thereof. Commercially available alkyl (meth)acrylate monomers may be, and typically are, mixtures of esters. Such mixtures are typically referred to, and are referred to herein, using a contracted version of the names of the ester species predominating in the mixture, e.g., xe2x80x9clauryl-myristyl methacrylatexe2x80x9d, xe2x80x9ccetyl-eicosyl methacrylatexe2x80x9d, xe2x80x9ccetyl-stearyl methacrylatexe2x80x9d, xe2x80x9cdodecyl-pentadecyl methacrylatexe2x80x9d.
In the process of the present invention, a reaction mixture of a diluent, appropriate relative amounts of appropriate respective alkyl (meth)acrylate monomers and an effective amount of a polymerization initiator is charged to a reaction vessel. preferably, the reaction vessel is equipped with a stirrer, a thermometer, a reflux condenser and a metering line.
The usefulness of the method of the present invention is not limited to the above described preferred copolymer compositions, i.e., the method provides improved demulsibility properties to known copolymers, e.g. poly(lauryl methacrylate/methyl methacrylate) copolymers, as well.
In a preferred embodiment, the reaction mixture includes from about 55 weight percent to about 99.5 weight percent of a first monomer selected from the group consisting of (C8-C15)alkyl (meth)acrylates and mixtures thereof and from about 0.5 weight percent to about 45 weight percent of a second monomer selected from the group consisting of (C1-C7)alkyl (meth)acrylates, (C16-C24)alkyl (meth)acrylates and mixtures thereof.
As used herein the terminology xe2x80x9c(C1-C7)alkyl (meth)acrylatesxe2x80x9d means an alkyl ester of (meth)acrylic acid having a straight or branched alkyl group of 1 to 7 carbon atoms per group and includes, e.g., methyl methacrylate, ethyl acrylate, propyl methacrylate, butyl acrylate, pentyl methacrylate, hexyl acrylate and heptyl methacrylate.
In a more highly preferred embodiment of the process, the monomers of the reaction mixture are selected so that the process provides a copolymer according to one of the above described preferred embodiments of the copolymer of the present invention.
In a preferred embodiment, those repeating units of the copolymer having structural formula (1) are derived from a (C8-C15)alkyl (meth)acrylate monomer, more preferably, a (C8-C15)alkyl methacrylate monomer.
As used herein, xe2x80x9c(C8-C15)alkyl (meth)acrylate monomerxe2x80x9d means an alkyl ester of (meth)acrylic acid having a straight or branched alkyl group of 8 to 15 carbon atoms per group, including, e.g., octyl methacrylate, nonyl methacrylate, decyl methacrylate, isodecyl methacrylate, undecyl methacrylate, lauryl methacrylate, lauryl acrylate, tridecyl methacrylate, myristyl methacrylate, pentadecyl methacrylate and mixtures thereof, e.g., lauryl-myristyl methacrylate, dodecyl-pentadecyl methacrylate.
In a preferred embodiment, those repeating units of the copolymer having structural formula (2) are derived from a (C4)alkyl (meth)acrylate monomer, more preferably, a (C4)alkyl methacrylate monomer.
As used herein, xe2x80x9c(C4)alkyl (meth)acrylate monomerxe2x80x9d is used synonymously with the terminology xe2x80x9cbutyl (meth)acrylatexe2x80x9d and means an alkyl ester of (meth)acrylic acid having a straight or branched alkyl group of 4 carbon atoms per group and includes, e.g., n-butyl acrylate, n-butyl methacrylate, iso-butyl methacrylate, t-butyl methacrylate.
In a preferred embodiment, those repeating units of the copolymer having structural formula (3) are derived from methyl (meth)acrylate monomer, more preferably, methyl methacrylate monomer.
In a preferred embodiment, those repeating units of the copolymer having structural formula (4) are derived from a (C16-C24)alkyl (meth)acrylate monomer, more preferably, a (C16-C24)alkyl methacrylate monomer.
As used herein, xe2x80x9c(C16-C24)alkyl (meth)acrylate monomerxe2x80x9d means an alkyl ester of (meth)acrylic acid having a straight or branched alkyl group of 16 to 24 carbon atoms per group, including, e.g., stearyl acrylate, stearyl methacrylate, cetyl methacrylate, heptadecyl methacrylate, nonadecyl methacrylate, eicosyl methacrylate and mixtures thereof, e.g., cetyl-stearyl methacrylate, cetyl-eicosyl methacrylate.
The diluent may be an inert hydrocarbon and is preferably a hydrocarbon lubricating oil which is compatible with or identical to the base oil in which the additive is to be subsequently employed. The mixture includes, e.g., from about 15 to about 400 parts by weight (pbw) diluent per pbw total monomers and, more preferably, from about 50 to about 200 pbw diluent per 100 pbw total monomers. As used herein, xe2x80x9ctotal monomersxe2x80x9d means the combined amount of all monomers in the reaction mixture.
Suitable polymerization initiators include those initiators which dissociate upon relatively mild heating, i.e., at temperatures up to about 100xc2x0 C., to yield a free radical.
In a preferred embodiment, the polymerization initiator is an initiator having a half-life of less than about 30 minutes at the intended reaction temperature. Those polymerization initiators having a half-life from about 1 minute to about 180 minutes at about 100xc2x0 C., e.g., 2,2xe2x80x2-azobis (2-methylbutanenitrile), 2,2xe2x80x2 azobis (2,4-dimethylpentanenitrile),1,1xe2x80x2-azobis(cyclohexanecarbonitrile), t-butyl peroctoate and mixtures thereof, are particularly preferred. More preferred are those polymerization initiators having a half life of about 5 minutes to about 30 minutes at about 90xc2x0 C., including, e.g., 2,2xe2x80x2-azobis (2-methylbutanenitrile), 2,2xe2x80x2 azobis (2,4-dimethylpentanenitrile).
The reaction mixture includes, e.g., from about 0.05 pbw to about 2.0 pbw polymerization initiator per 100 pbw total monomers and, more preferably, from about 0.1 pbw to about 1.0 pbw polymerization initiator per 100 pbw total monomers.
In a preferred embodiment, the reaction mixture includes a chain transfer agent. Suitable chain transfer agents include those conventional in the art, e.g., dodecyl mercaptan, ethyl mercaptan. Dodecyl mercaptan is preferred as the chain transfer agent. The selection of the amount of chain transfer agent to be used is based on the desired molecular weight of the polymer being synthesized in a manner conventional in the art. The reaction mixture typically includes, e.g., from about 0.05 pbw to about 1.9 pbw chain transfer agent per 100 pbw total monomers, and more preferably includes from about 0.1 pbw to about 0.8 pbw chain transfer agent per 100 pbw total monomers.
The reaction mixture is charged to the reaction vessel and heated with stirring, preferably under an inert, e.g., nitrogen, blanket to a temperature within a first reaction temperature range. Selection of limits of the first reaction temperature range is based on the initiator selected and the range includes those temperatures effective to rapidly dissociate the selected initiator to an upper limit of about 100xc2x0 C., e.g., from about 75xc2x0 C. to about 100xc2x0 C. In a preferred embodiment, the first reaction temperature range is from about 75xc2x0 C. to about 95xc2x0 C. The batch is then maintained at a temperature within the first reaction temperature range, with stirring, for a time period effective to allow copolymerization of the monomers in the reaction mixture, e.g., for about 2 hours to about 12 hours.
In a preferred embodiment, a portion, e.g., from about 25% to about 60%, of the reaction mixture is initially charged to the reaction vessel and heated to a temperature within the first reaction temperature range. The remaining portion of the reaction mixture is then fed into the reaction vessel with stirring and while maintaining the batch at a temperature within the first reaction temperature range over a time period of about 30 minutes to about 180 minutes. Following completion of the reaction mixture addition, the batch is maintained at a temperature within the first reaction temperature range for a holding period of up to about 4 hours.
In a preferred embodiment, an second portion of initiator of about 0.05 pbw to about 1.0 pbw polymerization initiator per 100 pbw total monomers is added to the reaction mixture subsequent to the holding period.
In one preferred embodiment, the second portion of initiator is added to the reaction mixture at a substantially continuous rate over a time period of about 30 minutes to about 180 minutes while maintaining the reaction mixture at a temperature within a second reaction temperature range, wherein the second reaction temperature range includes those temperatures, up to about 100xc2x0 C., e.g., from about 75xc2x0 C. to about 100xc2x0 C. and preferably from about 75xc2x0 C. to about 95xc2x0 C., effective to rapidly dissociate the added initiator.
In an alternate preferred embodiment, the second portion of polymerization initiator is added by periodically charging subportions of the second portion of initiator to the reaction vessel while maintaining the reaction mixture at a temperature within a second reaction temperature range, wherein the second reaction temperature range includes those temperatures, up to about 100xc2x0 C., e.g., from about 75xc2x0 C. to about 100xc2x0 C. and preferably from about 75xc2x0 C. to about 95xc2x0 C., effective to rapidly dissociate the added initiator.
In a preferred embodiment, the second portion of initiator is the same initiator as that in the initial reaction mixture and the first and second reaction temperature ranges are identical. Alternatively, the composition of the second portion of initiator may differ from the initiator present in the initial reaction mixture and, in the embodiment wherein subportions of initiator are periodically charged to the reaction vessel, the compositions of the respective subportions may differ from each other. Selection of the appropriate second reaction temperature range to be maintained following each addition of initiator is based on the respective half-life of each of the added portions of initiator in the manner described above.
The reaction mixture is held at the second reaction temperature range for a time period of about 30 minutes to about 180 minutes subsequent to the addition of the second portion of polymerization initiator to complete the polymerization reaction.
A viscous solution of the copolymer of the present invention in the diluent is obtained as the product of the reaction.
The above-discussed copolymers are each combined with a base oil, e.g., a paraffinic solvent neutral oil, in a conventional manner, i.e., by adding the copolymer to the base oil to form a solution of the additive in the base oil, to provide a hydraulic fluid composition having the desired viscometric properties.
In a preferred embodiment, a hydraulic fluid of the present invention includes from about 2 pbw to about 20 pbw viscosity index improving copolymer per 100 pbw base oil.
In a preferred embodiment, the copolymer is added to the base oil in the form of a relatively concentrated solution of the copolymer in a diluent, e.g., a solution of from about 100 pbw copolymer dissolved in from about 25 pbw to about 250 pbw of the hydrocarbon diluent used in the above described polymerization process.
The hydraulic fluid may include other conventional additives, e.g., antioxidants, anti-wear additives, in addition to the viscosity index improving copolymer of the present invention.